A central issue in understanding nervous system function is elucidation of the cellular events associated with intercellular communication and trophic factor action. The biological responses to trophic factors and other ligands is a consequence of a complex array of biochemical events which are initiated upon binding of these molecules to cell surface receptors. The intracellular mechanisms of signal transduction are mediated principally by the regulated phosphorylation of proteins which ultimately lead to biological responses. Heretofore, protein kinases were confined to one of two distinct and non-overlapping groups, the tyrosine kinases and the serine/threonine kinases. Recently a novel class of protein kinases has been discovered which has the capacity to phosphorylate all three hydroxy amino acids, termed dual specificity kinases. The suggestion that such enzymes may exist was considered heretical, and their recent identification has generated substantial excitement over their potential biological roles. The unique enzymatic activity of these kinases has implicated them as critical intermediates in intracellular signalling cascades. This proposal is focused on the characterization of CLK (an acronym for cdc2-like kinase), the first member of this family of dual specificity kinases to be identified. CLK is activated through autophosphorylation on tyrosine and serine residues. Of particular relevance to this proposal is the finding that CLK is expressed at high levels in the brain. We provide preliminary data demonstrating that expression of CLK in PC12 cells (which do not normally express this kinase) elicits the morphological differentiation of the cells in a manner analogous to the action of nerve growth factor (NGF). CLK has been shown to intervene at a very early point in the NGF-regulated intracellular signalling pathways and drives the activation of other hormonally-regulated protein kinases. We have also demonstrated that CLK directly phosphorylates and activates the S-6 kinase, p90rsk. We propose to determine the regional distribution and developmental regulation of CLK expression in the brain. The biochemical mechanisms by which CLK is activated will be investigated in vitro and in vivo. We will also investigate the other biological consequences of CLK action, including activation of other protein kinases, and stimulation of NGF-responsive gene transcription. Importantly, CLK has been shown to phosphorylate the microtubule associated protein, tau and the proto- oncogene product, c-Fos. The site at which these molecules are phosphorylated and potential functional consequences will be investigated.